The invention relates to seal clearances in rotary machines. More particularly, the invention relates to a method to modify the stationary casing in a manner to compensate for circumferentially non-uniform rotor movements.
Rotary machines include, but are not limited to, gas turbines and steam turbines. The moving part of the turbine is called a rotor and the fixed, non-moving part i. e. housings, casings etc. a stator. Usually, the rotor rotates within a stator assembly at very high speeds, powering a generator which in turn produces electricity or power. A steam turbine has a steam path that typically includes, in serial-flow relationship, a steam inlet, a turbine, and a steam outlet. A gas turbine has a gas path, which typically includes, in serial-flow relationship, an air intake (or inlet), a compressor, a combustor, a turbine, and a gas outlet (or exhaust nozzle). Gas or steam leakage, either out of the gas or steam path or into the gas or steam path, from an area of higher pressure to an area of lower pressure, is generally undesirable. For example, gas path leakage in the turbine or compressor area of a gas turbine, between the rotor of the turbine or compressor and the circumferentially surrounding turbine or compressor casing, will lower the efficiency of the gas turbine leading to increased fuel costs.
In practice, clearances between the rotating and stationary parts are often designed to be sufficiently large so that minimal contact occurs during the operation of the engine. However, overly generous clearances tend to promote undesirable leakages and decreased performance. In some machine designs, where reduced clearances have been designed for better efficiency, contact between rotor and stator is anticipated and accommodated by disposing a seal, such as a brush seal or an abradable seal, between these components. Abradable seals applied on the stationary parts of the gas or steam turbines have been used in order to allow the components from the rotating part (e.g. bucket tips, shaft teeth, etc.) to come into contact with the stator without suffering significant damage or wear. Contact between rotating elements and the abradable seal results in trenches worn into the abradable seal, creating a tight clearance between the two.
Effects such as thermal distortion of the casing and vibrations due to rotor dynamics often cause the path of relative rotor motion to become circumferentially non-uniform with respect to the stator. This non-uniformity of motion can lead to substantial contact in preferential, localized areas of the stator, resulting in undesirable amounts of component wear. A number of approaches have been tried to compensate for this non-uniform motion and resultant prevention of contact. Conventionally, machine parts have been machined circular and assembled with generous uniform clearances to prevent contact. The large clearances allow for more gas or steam to escape, however, which degrades system performance. In certain cases, parts are machined off-center to provide non-uniform clearances, but this complicates their fabrication and significantly boosts costs. In some steam turbines, seals are segmented into 4, 6, 8, or more segments, and the segments are each machined to a different diameter. This greatly complicates turbine assembly because individual parts must be tracked and assembled insitu in their specific circumferential locations. Therefore, what is needed is a cost-effective stator component that is capable of producing non-uniform rotor clearances. A further need is for efficient methods for making such components.